1. Field of the Invention
The present invention relates to an air-fuel ratio control system for a multi-cylinder internal combustion engine and, more particularly, relates to an air-fuel ratio control system that is employed in a multi-cylinder internal combustion engine and is designed to reduce an amount of fuel injected into some cylinders during engine start and to activate a catalyst at an early stage.
2. Description of the Related Art
According to a conventionally employed control method intended to activate a catalyst at an early stage, fuel injection into some cylinders of a multi-cylinder internal combustion engine is stopped during a cold start thereof. Thus, exhaust gas that is discharged from those cylinders is supplied to a catalytic converter as secondary air, which exhibits a high concentration of oxygen and contains no fuel at all. As a result, the catalyst is urged to further promote reactions for oxidizing HC and CO.
For example, Japanese Patent Application Laid-Open No. HEI 7-83148 discloses an art intended to activate a catalyst at an early stage while maintaining good idling stability. According to this art, during cold start of a multi-cylinder internal combustion engine with a plurality of cylinders, some of those cylinders are subjected to rich-state operation wherein the air-fuel ratio assumes a value on the rich side, and other cylinders are subjected to lean-state operation wherein the air-fuel ratio assumes a value on the lean side. Lean-state operation of those cylinders is accomplished by either reducing an amount of is fuel injected thereto or completely stopping fuel injection thereto. While the ignition timing is retarded with respect to those cylinders which are to be subjected to rich-state operation, it is advanced with respect to those cylinders which are to be subjected to lean-state operation. These ignition timing correcting operations are alternately performed in each of the cylinders.
However, according to the air-fuel ratio control system for the multi-cylinder internal combustion engine disclosed in Japanese Patent Application Laid-Open No. HEI 7-83148, some of the cylinders undergo lean-state operation, which results in a drop in temperature of an exhaust system. Hence, it takes a long time for the catalyst to reach a temperature at which the catalyst starts to be activated (hereinafter referred to as an activation start temperature), that is, a temperature at which the catalyst temperature starts rising sharply (hereinafter referred to as a catalytic reaction promoting temperature).